Inductive conductivity sensors are usually evaluated with electrical current measuring methods, wherein the output current of the measuring coil of the inductive, conductivity sensor is measured by a circuit whose input impedance lies near zero. Since an undesired residual coupling of the coils exists in every inductive, conductivity sensor, such residual coupling, which is also referred to as “air set,” must be measured once at the initial start-up of the inductive, conductivity sensor. This is done while the conductivity sensor is in the air, where an ideal conductivity sensor issues no measurement signal, while a real conductivity sensor, in contrast, has a non-zero signal. During the running of the measurement operation of the conductivity sensor, measurement results are corrected with the residual coupling so ascertained. The residual coupling is usually measured only a single time, while the inductive, conductivity sensor remains in operation for months or years. In this time, the inductances of the conductivity sensor are subject to an aging related drift, so that the residual coupling of the conductivity sensor changes. Since the residual coupling is temperature dependent, a further drift of the residual coupling brings about a reduction in the accuracy of measurement of the conductivity sensor.
DE 41 16 468 A1 discloses a method for determining the functional ability of an inductive, conductivity sensor, in which, supplemental to a measuring conductor loop, which has an equivalent impedance of the medium, an additional conductor loop is present, which has a variable impedance.
It is known from DE 102 86 79 A1 that, for measuring conductivity, the conductivity sensor, including the conductor loop, is placed in the measured medium and the variable impedance is changed. Since the equivalent impedance of the medium and the variable impedance act on the measuring coil of the conductivity sensor with a 180° phase shift, the magnitudes of the equivalent impedance of the medium and of the variable impedance cancel exactly, if the output voltage of the conductivity sensor is equal to zero. Thus, the magnitude of the variable impedance corresponds equally to the value of the equivalent impedance of the medium. In order to achieve an exact measurement result, this method requires that the variable impedance is exact over a broad adjustment range of 6 to 7 orders of magnitude, and that it has a linear transfer function, as well as temperature stability and long term stability. These requirements are not implementable in practice, so that the results of this measuring method are not sufficiently exact.